Process for preparing vinyl acetate using a supported palladium and gold catalyst

ABSTRACT

A process for the production of a catalyst for preparing vinyl acetate in the gas phase from ethylene, acetic acid and oxygen or oxygen-containing gases which catalyst comprises palladium and/or its compounds, gold and/or its compounds and also alkali metal compounds on a particulate, porous support obtained by 
     a) impregnating the support with soluble palladium and gold compounds, 
     b) converting the soluble palladium and gold compounds into insoluble palladium and gold compounds by addition of an alkaline solution to the support. 
     c) reducing the insoluble palladium and gold compounds on the support with a reducing agent in the liquid or gaseous phase, 
     d) impregnating the support with at least one soluble alkali metal compound and 
     e) finally drying the support at a maximum of 150° C., 
     wherein the catalyst is brought into contact with at least one peroxidic compound in step b).

PRIOR APPLICATION

This application is a division of U.S. Patent application Ser. No.09/203,689 filed Dec. 2, 1998, now U.S. Pat. No. 5,972,824.

A catalyst comprising palladium and/or its compounds, gold and/or itscompounds and also at least one alkali metal compound, a process forproducing it and its use for preparing vinyl acetate in the gas phasefrom acetic acid, ethylene and oxygen or oxygen-containing gases.

STATE OF THE ART

It is known from the prior art that vinyl acetate can be prepared in thegas phase from ethylene, oxygen and acetic acid in the presence ofcatalysts which comprise palladium, gold and alkali metal compounds on aporous support material such as silicon dioxide.

The distribution of the noble metals on the support material is ofparticular importance for the activity and selectivity of thesecatalysts. Since the reactants in the reaction to be catalyzed cannotreadily diffuse into the intermediate or inner regions of the poroussupport material, the reaction takes place essentially only on theoutermost or surface regions of the catalyst. Thus, the metal componentspresent in the interior or in the intermediate regions of the support donot contribute significantly to the reaction mechanism, which leads to areduction in productivity of the catalyst based on the weight of thenoble metals.

In the development of effective catalysts for vinyl acetate production,efforts have therefore been directed at providing catalysts in which thecatalytically active noble metals are present in a shell on the supportparticles while the core of the support particles is largely free ofnoble metals. Such catalysts can in principle be produced byimpregnation of the support material with soluble noble metal compounds,subsequent precipitation of insoluble noble metal compounds on thesupport by addition of alkaline compounds and final reduction to thenoble metals.

U.S. Pat. No. 4,048,096 describes a process for producing a palladium-,gold- and potassium-containing catalyst for vinyl acetate production.The catalyst support is first impregnated with a solution comprising amixture of the dissolved palladium and gold salts. It is essential tothat invention that the solution has the same volume as the pores of thesupport material in the dry state. During the impregnation step, thesupport particles are kept in motion in a rotating vessel. Without priordrying of the impregnated support, the noble metal salts on the supportparticles are subsequently converted into insoluble compounds byaddition of alkalis and are thus fixed to the support particles. Thepalladium and gold compounds are reduced to the corresponding metals bya final treatment with a reducing agent. Application of an alkali metalcompound in a further impregnation step gives a catalyst which has thedesired shell structure and comprises palladium and gold in a thicknessof 0.5 mm on the surface of the support material.

U.S. Pat. No. 3,775,342 also describes the production of a palladium-,gold- and potassium-containing catalyst for vinyl acetate production. Inthis process, the support material is treated in any order with twosolutions of which one comprises the dissolved palladium and gold saltsand the other an alkaline substance. After treatment with the firstsolution, the support is dried in an intermediate step before beingbrought into contact with the second solution. The volume of bothsolutions corresponds to the pore volume of the support material.

Furthermore, U.S. Pat. No. 5,332,710 discloses the production of acatalyst for preparing vinyl acetate, in which the insoluble noble metalsalts are likewise precipitated on the support particles by addition ofalkalis. For this purpose, the support particles are immersed in thealkaline solution and are subjected to rotary motion from thecommencement of the precipitation for at least half an hour in a drum.This process is known as "rotation-immersion."

In the preparation of vinyl acetate, the catalysts produced as describedin the above-mentioned process frequently lead to undesirably highformation of degradation products and by-products, e.g. carbon dioxide,thus adversely affecting activity and selectivity of the overallreaction.

OBJECTS OF THE INVENTION

Since vinyl acetate is a volume product produced on a large industrialscale, it is an object of the invention to provide a catalyst which hasa further improved activity and selectivity in the preparation of vinylacetate in the gas phase.

This and other objects and advantages of the invention will becomeobvious from the following detailed description.

THE INVENTION

The invention provides a process for producing a catalyst for thepreparation of vinyl acetate in the gas phase from ethylene, acetic acidand oxygen or oxygen-containing gases, which catalyst comprisespalladium and/or its compounds, gold and/or its compounds and alsoalkali metal compounds on a particulate, porous support produced by

a) impregnating the support with soluble palladium and gold compounds,

b) converting the soluble palladium and gold compounds into insolublepalladium and gold compounds by addition of an alkaline solution to thesupport,

c) reducing the insoluble palladium and gold compounds on the supportwith a reducing agent in the liquid or gaseous phase,

d) impregnating the support with at least one soluble alkali metalcompound and

e) finally drying the support at a maximum of 150 C., the improvementcomprising the catalyst is brought into contact with at least oneperoxidic compound in step b).

The invention also provides a catalyst for preparing vinyl acetate inthe gas phase from ethylene, acetic acid and oxygen or oxygen-containinggases, which comprises palladium and/or its compounds, gold and/or itscompounds and also alkali metal compounds on a particulate, poroussupport obtained by the above-described process.

The invention further provides a process for preparing vinyl acetate inthe gas phase from ethylene, acetic acid and oxygen and/oroxygen-containing gases in the presence of a catalyst obtained by theabove-described process. In the preparation of vinyl acetate, thecatalysts of the invention surprisingly lead both to an improvedactivity and to a higher selectivity of the reaction.

The support particles of the catalyst of the invention can have anygeometric shape, for example spheres, pellets, cylinders, rings or starswith a regular or irregular configuration. The dimensions of the supportparticles, i.e. the diameter or the length and thickness are generallyfrom 1 to 10 mm, particularly from 3 to 9 mm. Preference is given tousing spherical support particles having a diameter of from 4 to 8 mm.

Supports which can be used are the known inert support materials such assilica, aluminum oxide, alumino-silicates, silicates, titanium oxide,zirconium oxide, titanates, silicon carbide and carbon. Other suitablesupport materials are the pyrogenic silicas obtained by flame hydrolysisof silicon tetrachloride or the pyrogenic SiO₂ --M_(x) O_(y) mixtureobtained by flame hydrolysis of silicon tetrachloride and another metalchloride such as aluminum chloride (U.S. Pat. No. 3,939,199 and EP-A-0723 810). Preference is given to using silica (SiO₂), baddeleyite (ZrO₂)and SiO₂ --Al₂ O₃ mixtures as support material. In the case of thepyrogenic support materials, the pressed bodies described in DE-A-38 03895 and DE-A-39 12 504 are particularly suitable.

To be suitable as support material, it is critical that the materialretains its mechanical strength under the reaction conditions of thecatalytic process for preparing vinyl acetate, particularly in thepresence of acetic acid.

Particularly suitable supports are those of the above-mentioned typehaving a specific surface area of from 50 to 400 m² /g (measured by theBET method) and a mean pore radius of from 50 to 2000 Å (measured bymeans of mercury porosimetry).

In step a) of the process of the invention, the impregnation step, thesupport particles are impregnated with the dissolved palladium and goldcompounds. Suitable palladium and gold compounds are all salts andcomplexes which are soluble in the solvents described below, can beprecipitated as hydroxide or oxide and in the finished catalyst,possibly after a washing step, leave no materials which impairperformance of the catalyst.

Examples of suitable palladium compounds are palladium(II) chloride,sodium and potassium chloropalladate(II), palladium(II) nitrate,nitrite, sulfate, oxalate, acetylacetonate or acetoacetate and hydratedpalladium(II) oxide. It is also possible to use palladium salts ofaliphatic monocarboxylic acids of 2 to 5 carbon atoms, preferablypalladium(II) acetate. Gold(III) chloride, gold(III) acetate,tetrachloroauric (III) acid and its alkali metal salts can be used assoluble gold compounds. In general, these compounds are used in suchamounts that the finished catalyst comprises from 2 to 14 g/l,preferably from 4 to 8 g/l, of palladium and from 1 to 8 g/l, preferablyfrom 2 to 5 g/l, of gold.

Suitable solvents for the palladium and gold compounds and also for thealkali metal compounds to be applied in step d) are all compounds inwhich the salts chosen are soluble and which are easy to remove again inan optional drying step after the impregnation. Particularly suitablesolvents are water and unsubstituted carboxylic acids of 2 to 10 carbonatoms, e.g. acetic acid, propionic acid, n- and iso-butyric acids and n-and iso-valeric acids. owing to its favorable physical properties andalso for economic reasons; the preferred carboxylic acid is acetic acid.

It is advantageous to use an additional solvent when the palladium andgold compounds are not sufficiently soluble in the carboxylic acid used.Thus, for example, palladium(II) chloride dissolves significantly betterin aqueous acetic acid than in glacial acetic acid. Suitable additionalsolvents are those which are inert and at the same time are misciblewith the carboxylic acid, e.g. water, ethers such as tetrahydrofuran ordioxane and hydrocarbons such as benzene.

In the impregnation of the support material, it is possible to use aplurality of salts of each of the metals to be applied, but preferenceis given to using only one salt per metal.

The impregnation of the support material with the soluble palladium andgold compounds in step a) can be carried out using a solution whichsimultaneously comprises all soluble palladium and gold compounds. Here,the support material can be impregnated once or a plurality of timeswith this solution. Since the amount of palladium and gold compoundsused should be identical in single and multiple impregnation, the totalvolume of the solution should be divided appropriately in the case ofmultiple impregnation. Preference is given to a single impregnation withthe total volume of the solution.

In an alternative embodiment, the impregnation of the support materialcan also be carried out using two separate solutions of which onecontains the palladium compounds and the other contains the goldcompounds. In this case, the two solutions can be brought into contactwith the support material either simultaneously or else in any order. Inthe latter case, the support has to be dried after impregnation with thefirst solution.

For effective impregnation, the total volume of the noble metal saltsolution or of the two noble metal salt solutions should be about90-100%, preferably 95-100% and in particular 98-99%, of the pore volumeof the support material in the dry state. In practice, it is alsopossible to cover the support particles with an excess of the noblemetal salt solution and subsequently to pour away or filter off theexcess solution. However, preference is given to adding only theabove-described amount of solution corresponding approximately to thepore volume of the catalyst support.

It has been found to be advantageous to keep the support particles inmotion during the impregnation to achieve intimate mixing. This can bedone by means of a rotating or shaken flask or a mixing drum. Therotational speed or in general terms, the intensity of the motion shouldbe sufficient to achieve complete wetting of the support particles withthe impregnation solution but must not be so great that appreciableabrasion of the support material occurs.

The catalyst can subsequently be dried at temperatures of at most 150°C., preferably 80-150° C. and more preferably 100-150° C. This dryingprocedure can be carried out, for example, in a stream of hot air in afan-forced drier or in a drying oven in a stream of inert gas,particularly a stream of nitrogen or carbon dioxide. Drying is carriedout at atmospheric pressure or under reduced pressure, preferably0.01-0.08 MPa.

In step b), the fixing step, the soluble palladium and gold compoundspresent on the support particles are converted into insoluble compoundswith an alkaline solution and are thus fixed to the support. It isassumed that the insoluble compounds are the hydroxides and/or oxides ofthe noble metals.

Suitable alkaline solutions are all solutions which are able to convertthe soluble palladium and gold compounds into insoluble compounds.Example of alkaline reagents which can be used are alkali metalhydroxides, alkali metal silicates and alkali metal carbonates.Preference is given to an aqueous solution of the alkali metalhydroxides, particularly of potassium or sodium hydroxide. Solutionscontaining boron compounds can also be used as alkaline solutions. Here,aqueous solutions of sodium tetraborate decahydrate (borax), potassiumtetraborate or mixtures of alkali metal hydroxide and boric acid areparticularly suitable. The alkaline solution can have buffer properties.

The amount of alkaline compound present in the aqueous solution isadvantageously selected so that it is at least sufficient for thestoichiometric reaction with the soluble palladium and gold compoundsapplied. However, it is also possible to use an excess of the alkalinecompound, usually 1-10 times the stoichiometrically required amount.

It is essential to the process of the invention that the catalyst isbrought into contact with at least one peroxidic compound in step b).This peroxidic compound can be, for example, a perborate, preferablysodium perborate, a percarbonate, preferably sodium percarbonate, aperoxodisulfate, preferably sodium peroxodisulfate, or hydrogenperoxide.

One possible embodiment comprises adding the peroxidic compound to thealkaline solution which already comprises one of the above-mentionedalkaline substances, preferably an alkali metal hydroxide. In analternative embodiment, a second, separate solution comprising theperoxidic compound can be used in step b) in addition to the alkalinesolution. In this case, the impregnated catalyst support, as describedbelow, is first brought into contact with the alkaline solution andsubsequently treated with the aqueous solution of the peroxidic compoundbefore the reduction is carried out in step c). Since some of theperoxidic compounds mentioned are themselves alkaline, e.g. theperborates and percarbonates, it is also possible, in a third andpreferred embodiment, for the alkaline solution to be used in step b) tocomprise only the peroxidic compound which is simultaneously alkaline.

It has been found to be useful to heat the solution which comprises theperoxidic compounds to a maximum of 90° C., preferably to 60-85° C.,before addition to the impregnated catalyst support.

In all three embodiments, the peroxidic compound is used in a 1-20-fold,preferably 5-10-fold, excess based on the concentration of the noblemetal salt. It is found that contact of the impregnated catalyst supportwith at least one peroxidic compound in the fixing step b) leads to somereduction of the noble metals.

Two methods I and II which are suitable for carrying out the fixing stepb) and can be employed in the production of the catalyst of theinvention are described below.

In method I, the support material impregnated in step a) is placed for asufficient time in an alkaline solution whose concentration is such thatthe desired, insoluble noble metal compounds are precipitated. Inaddition, the volume of the alkaline solution is selected so that it issufficient to completely cover and immerse the impregnated supportparticles. Furthermore, the impregnated support particles immersed inthe alkaline solution are subjected to rotary motion commencing with theprecipitation of the insoluble palladium and gold compounds for at leasthalf an hour, preferably one hour and at most up to 4 hours. This fixingmethod is known as "rotation-immersion" and is described in detail inU.S. Pat. No. 5,332,710, which is hereby incorporated by reference.

In this variant I, the additional treatment of the catalyst support withthe peroxidic compound can be carried out as described in the threeabove-mentioned embodiments.

If the method II described below is employed for fixing the palladiumand gold compounds to the support particles, the support which has beenimpregnated in step a) should be dried before the fixing step b).

In method II, the fixing step b) comprises at least two separate stagesof treatment with the alkaline fixing solution. In the first fixingstage, the impregnated and then dried support is brought into contactwith the alkaline fixing solution. The volume of this first fixingsolution corresponds to the pore volume and thus the absorptive capacityof the support material in the dry state. The amount of alkalinecompounds present therein should be such that the molar ratio of alkalimetal from the alkaline compound to anions from the soluble metal saltis in the range from 0.7:1 to 2:1. For absorption on the supportparticles, the alkaline fixing solution is poured onto the supportparticles and they are then left to stand for up to 24 hours, preferably2-8 hours.

In this method II, the second fixing stage can be carried out in twovariants A) and B). In both variants, the molar ratio of the alkalimetal from the alkaline compound to the anion from the metal salt isfrom about 0.2:1 to 2:1 in the fixing solution.

In variant A) of method II, the undried support particles are broughtinto contact with the second fixing solution whose volume should atleast just cover the supports. For absorption on the support particles,the alkaline fixing solution is poured onto the support particles andthey are then left to stand for up to 16 hours, but at least 2 hours andpreferably at least 4 hours.

In variant B), the support material after contact with the first fixingsolution is, in the second step, treated by the rotation-immersionprocess of U.S. Pat. No. 5,332,710. Here, the support material isimmersed in the alkaline fixing solution of the second step and at thesame time subjected to rotary motion. This rotation should continue forat least half an hour, preferably one hour and at most up to 4 hours.

Regardless of whether variant A) or B) is employed, the treatment in thesecond fixing step can be equivalent to the treatment in the first stagein that a fixing solution of the same concentration is used and thevolume of the second fixing solution likewise corresponds to the porevolume and thus the absorptive capacity of the support material in thedry state. The total molar ratio of alkali metal to anion from the metalsalt for both fixing stages together is preferably in the range from1.1:1 to 3.3:1.

In method II, the additional treatment of the catalyst support with theperoxidic compound can in principle be carried out in either of the twofixing stages, but it is preferably carried out in the second fixingstage as described in the three above-mentioned embodiments.

After the fixing step of method I or the last fixing step of method II,the supports can be washed with water, preferably with distilled water,to remove any interfering anions, e.g. chlorides, which originate fromthe impregnation step, have been set free by the precipitation of thenoble metals and are still present on the support material. This washingprocedure also removes any excess of alkaline compound which may stillbe present.

The catalyst can then be dried at temperatures of at most 150° C.,preferably 80-150° C. and more preferably 100-150° C. This dryingprocedure can be carried out, for example, in a stream of hot air in afan-forced drier or else in a drying oven in a stream of inert gas,particularly in a stream of nitrogen or carbon dioxide. Drying iscarried out at atmospheric pressure or under reduced pressure,preferably 0.01-0.08 MPa.

Such a drying procedure is advantageous at this point particularly whenthe reduction step c) described below is carried out in the gas phase.In contrast, prior drying is not necessary if the reduction is carriedout in the liquid phase.

In step c), the support together with the insoluble palladium and goldcompounds deposited thereon is treated with a reducing agent to convertthe precipitated palladium and gold compounds into the metallic form.This reduction can be carried out in the liquid phase at a temperatureof 0-90° C., preferably 15-25° C.

The reducing agent used here is, for example, hydrazine, formic acid oran alkali metal borohydride, preferably sodium borohydride. As analternative, it is also possible to carry out the reduction in the gasphase using hydrogen, ethylene, propylene, isobutylene, butylene orother olefins as reducing agent. In this case, it is advantageous tocarry out the reduction at an increased temperature of 40-260° C.,preferably 70-200° C. It is also advantageous to dilute the reducingagent with an inert gas. The inert gas used can be, for example,nitrogen, carbon dioxide or a noble gas. Such a reducing agent/inert gasmixture usually contains 0.01-50% by volume, preferably 0.5-20% byvolume, of reducing agent.

Regardless of whether the reduction is carried out in the liquid or gasphase, the reducing agent should be added in an excess, based on thecatalyst to be reduced, so as to ensure that all the insoluble noblemetal compound is converted into the metallic form.

After the reduction, the support particles can be washed once again or aplurality of times, preferably with distilled water, to removeinterfering anions, e.g. chlorides, and residues of the alkalinesolution used. The washing procedure can also serve to remove residuesof the reducing agent from step c).

Subsequently, the catalyst is dried again under drying conditions whichshould be similar to those of a drying step after the fixing step b).

Finally, the addition of at least one alkali metal compound isnecessary. The catalyst is therefore impregnated with an aqueoussolution of an alkali metal compound in step d). Alkali metal compoundswhich can be used are sodium, potassium, rubidium or cesium compounds;preference is given to potassium compounds.

Suitable anions of these alkali metal compounds are, in particular,carboxylates, especially acetates or propionates. Particular preferenceis given to using potassium acetate. However, it is also possible to usecompounds which liberate alkali metal acetates under the reactionconditions, i.e. the alkali metal hydroxides, oxides or carbonates whenacetic acid is used as solvent. This impregnation is carried out, inprinciple, in the same way as the impregnation of the support materialin step a). The solvents which can be used are subject to the sameconditions and definitions as in the case of the solutions inimpregnation step a). The alkali metal compound is used in such anamount that the catalyst after the drying step described below contains0.1-10% by weight of alkali metal, preferably 1-4% by weight of alkalimetal, in particular potassium, based on the total mass of the catalyst.

Finally, the catalyst is, in step e), dried at temperatures of at most150° C., preferably 80-150° C. and more preferably 100-150° C. Thisdrying procedure can be carried out, for example, in a stream of hot airin a fan-forced drier or in a drying oven in a stream of inert gas,particularly in a stream of nitrogen or carbon dioxide. Drying iscarried out at atmospheric pressure or under reduced pressure,preferably 0.01-0.08 MPa.

The catalyst obtained by steps a) to e) of the process of the inventionand the treatment with the peroxidic compounds essential to theinvention in step b) comprises, based on the total mass of the catalyst,0.2-2.5% by weight, preferably 0.6-1.5% by weight, of palladium,0.2-2.5% by weight, preferably 0.3-1.0% by weight, of gold and 0.1-10%by weight of alkali metal, preferably 1.0-4.0% by weight of alkalimetal, in particular potassium.

Vinyl acetate is prepared by passing acetic acid, ethylene and oxygen oroxygen-containing gases at temperatures of from 100 to 220° C.,preferably from 120 to 200° C., and pressures of from 0.1 to 2.5 MPa,preferably from 0.1 to 2 MPa, over the catalyst of the invention.Unreacted components can be circulated. In some cases, dilution withinert gases such as nitrogen or carbon dioxide is also advantageous.Carbon dioxide is particularly suitable for dilution in a circulationmode of operation since it is in any case formed during the reaction.

It has been found to be useful to carry out the preparation of the vinylacetate in a stirred reactor, a Berty reactor, in circulation mode inthe gas phase at a constant oxygen conversion of about 45%. The reactoris first charged with the catalyst. Subsequently, a measured amount ofacetic acid and also ethylene and oxygen diluted with nitrogen isintroduced and the temperature is increased to the desired value using aheating mantle. The reaction is usually stopped after about 18 hours, aslong as it has been possible to set a temperature at which the oxygenconversion is constant at 45%. The composition of the product mixture isdetermined by means of gas chromatography.

The higher selectivity and activity achievable using the catalysts ofthe invention can in practice be utilized in two ways:

Firstly, to produce a larger amount of vinyl acetate per unit volume andunit time in existing plants while retaining all other reactionconditions, owing to the higher selectivity, the product mixture takenfrom the reactor has a higher proportion of vinyl acetate and containsless by-products, particularly carbon dioxide. In this way, the work-up,i.e. the isolation of the vinyl acetate, is made easier because, forexample, the amount of carbon dioxide to be separated off is lower andaccordingly the loss of entrained ethylene associated with the removalof carbon dioxide drops. This leads to a saving in starting material.The principles of the work-up of the product mixture after thepreparation of vinyl acetate are described, for example, in EP-A-0 423658.

The second possible way of utilizing the improved properties of thecatalysts of the invention is to lower the reaction temperature in thepreparation of vinyl acetate while maintaining the same space-timeyield. A lower reaction temperature in turn has a positive effect on thetotal operational life of the catalyst.

In the following examples, there are described several preferredembodiments to illustrate the invention. However, it should beunderstood that the invention is not intended to be limited to thespecific embodiments.

The catalysts in Examples 1-5 are produced using silica based onbentonite as support material which is the KA-160 support fromSud-Chemie. Spheres having a diameter of 7 mm were employed in Examples1-4 and 6-8 and spheres having a diameter of 5 mm were employed inExample 5.

EXAMPLE 1

5.37 g (0.0164 mol) of K₂ PdCl₄ and 3.36 g (0.0089 mol) of KAuCl₄ weredissolved together in 80 ml of demineralized water. All of this solutionwas, with gentle motion, applied to 131 g of the support material whichhad been pretreated in this way was placed in a solution of 18.31 g(0.12 mol) of sodium perborate tetrahydrate (NaBO₃ ·4H₂ O) in 300 ml ofdistilled water. The total reaction mixture was rotated on a rotaryevaporator at a speed of 5 revolutions per minute for 3.5 hours at 85°C. to complete the reaction. The reaction mixture was allowed to standfor about 12 hours and then was washed free of chloride withdemineralized water. The freedom from chloride was checked with thesilver nitrate test for chloride ions in aqueous solution. The materialwas then dried for 2 hours at 100° C. It was shown by photoelectronspectroscopy that after this step, the noble metal shell formedcomprised metallic gold and palladium in the oxidation state +2.Subsequently, the noble metals were reduced completely using dilutedethylene (5% in nitrogen). For this purpose, the gas mixture was passedover the catalyst for 5 hours at 150° C. 10 g of potassium acetate werethen dissolved in 75 ml of distilled water and added a little at a timeto the catalyst and the latter was dried once more for 2 hours at 100°C.

EXAMPLE 2

5.37 (0.0164 mol) of K₂ PdCl₄ and 1.92 g (0.0051 mol) of KAuCl₄ weredissolved together in 80 ml of demineralized water and all of thissolution was, with gentle motion, applied to 131 g of the supportmaterial. The support which had been pretreated in this way was placedin a solution of 14.92 g (0.097 mol) of sodium perborate tetrahydrate(NaBo₃ ·4H₂ O) in 300 ml of distilled water and the total reactionmixture was rotated on a rotary evaporator at a speed of 5 revolutionsper minute for 3.5 hours at 85° C. to complete the reaction. Thereaction mixture was allowed to stand for about 12 hours and was thenwashed free of chloride with demineralized water. The further procedurewas as described in Example 1.

EXAMPLE 3

12.88 g (0.0349 mol) of K₂ PdCl₄ and 4.6 g (0.0122 mol) of KAuCL₄ weredissolved together in 192 ml of demineralized water and all of thissolution was, with gentle motion, applied to 314.4 g of the supportmaterial. The support which had been pretreated in this way was placedin a solution of 35.8 g (0.23 mol) of sodium perborate tetrahydrate(NaBO₃ ·4H₂ O) in 720 ml of distilled water and the total reactionmixture was rotated on a rotary evaporator at a speed of 5 revolutionsper minute for 3.5 hours at 85° C. to complete the reaction. Thereaction mixture was allowed to stand for about 12 hours and was thenwashed free of chloride with distilled water. The further procedure wasas described in Example 1.

EXAMPLE 4

12.88 (0.0349 mol) of K₂ PdCl₄ and 8.06 g (0.0214 mol) of KAuCl₄ weredissolved together in 192 ml of demineralized water and all of thissolution was, with gentle motion, applied to 314.4 g of the supportmaterial. The support which had been pretreated in this way was placedin a solution of 35.8 g (0.23 mol) of sodium perborate tetrahydrate(NaBO₃ ·4H₂ O) in 720 ml of distilled water and the total reactionmixture was rotated on a rotary evaporator at a speed of 5 revolutionsper minute for 3.5 hours at 85° C. to complete the reaction. Thereaction mixture was allowed to stand for about 12 hours and was thenwashed free of chloride with distilled water. The further procedure wasas described in Example 1.

EXAMPLE 5

5.37 g (0.0164 mol) of K₂ PdCl₄ and 3.36 g (0.0089 mol) of KAuCl₄ weredissolved together in 90 ml of demineralized water and all of thissolution was, with gentle motion, applied to 147.5 g of the supportmaterial. The support which had been pretreated in this way was placedin a solution of 18.31 g (0.12 mol) of sodium perborate tetrahydrate(NaBO₃ ·4H₂ O) in 300 ml of distilled water and the total reactionmixture was rotated on a rotary evaporator at a speed of 5 revolutionsper minute for 3.5 hours at 85° C. to complete the reaction. Thereaction mixture was allowed to stand for about 12 hours and was thenwashed free of chloride with distilled water. The further procedure wasas described in Example 1.

EXAMPLE 6

7.67 g (0.0235 mol) of K₂ PdCl₄ and 3.84 g (0.0102 mol) of KAuCl₄ weredissolved together in 90 ml of demineralized water and all of thissolution was, with gentle motion, applied to 133.75 g of the supportmaterial. The support which had been pretreated in this way was placedin a solution of 23.85 g (0.16 mol) of sodium perborate tetrahydrate(NaBO₃ ·4H₂ O) in 300 ml of distilled water and the total reactionmixture was rotated on a rotary evaporator at a speed of 5 revolutionsper minute for 3.5 hours at 85° C. to complete the reaction. Thereaction mixture was allowed to stand for about 12 hours and was thenwashed free of chloride with distilled water. The further procedure wasas described in Example 1.

EXAMPLE 7

2.69 g (0.0082 mol) of K₂ PdCl₄ and 0.96 g (0.0025 mol) of KAuCl₄ weredissolved together in 40 ml of demineralized water and all of thissolution was, with gentle motion, applied to 65.5 g of the supportmaterial. The support which had been pretreated in this way was placedin a solution of 1.89 g (0.034 mol) of potassium hydroxide in 150 ml ofdistilled water and was rotated in this solution on a rotary evaporatorat a speed of 5 revolutions per minute for 2.5 hours at roomtemperature. Then, the reaction mixture was allowed to stand for about12 hours and then the support was separated from the KOH-solution. Thewet support material was then brought into contact with 150 ml of anaqueous solution which contained 18.84 g (0.12 mol) of sodiumpercarbonate and which had been heated at first to 60° C. Then, thereaction mixture was heated immediately on a water bath to 85° C. tocomplete the reaction. Subsequently, the total reaction mixture wasrotated the reaction. Subsequently, the total reaction mixture wasrotated on a rotary evaporator at a speed of 5 revolutions per minutefor 3.5 hours at 85° C. to complete the reaction. The reaction mixturewas allowed to stand for about 12 hours and was then washed free ofchloride with demineralized water. The further procedure was asdescribed in Example 1.

EXAMPLE 8

2.69 g (0.0082 mol) of K₂ PdCl₄ and 0.96 g (0.0025 mol) of KAuCl₄ weredissolved together in 40 ml of demineralized water and all of thissolution was, with gentle motion, applied to 65.5 g of the supportmaterial. The support which had been pretreated in this way was placedin a solution of 1.15 g (0.029 mol) of sodium hydroxide and 9.94 g of a30% strength hydrogen peroxide solution (corresponds to 0.088 mol (2.98g) of hydrogen peroxide) in 150 ml of distilled water and the totalreaction mixture was rotated on a rotary evaporator at a speed of 5revolutions per minute for 3.5 hours at 85° C. to complete the reaction.The reaction mixture was allowed to stand for about 12 hours and wasthen washed free of chloride with demineralized water. The furtherprocedure was as described in Example 1.

COMPARATIVE EXAMPLE

5.37 g (0.0164 mol) of K₂ PdCl₄ and 1.92 g (0.0051 mol) of KAuCl₄ weredissolved together in 87 ml of demineralized water and all of thissolution was, with gentle motion, applied to 133.75 g of the supportmaterial. The support which had been pretreated in this way was placedin a solution of 19.22 g (0.05 mol) of sodium tetraborate decahydtrate(Na₂ B₄ O₇ ·10H₂ O) in 300 ml of distilled water and the total reactionmixture was rotated on a rotary evaporator at a speed of 5 revolutionsper minute for 3.5 hours at 85° C. to complete the reaction. Thereaction mixture was allowed to stand for about 12 hours and was thenwashed free of chloride with demineralized water. The further procedurewas as described in Example 1.

To examine the performance of the catalysts described in the preparationof vinyl acetate, tests were carried out in a Berty reactor and theresults are summarized in the table:

    ______________________________________                                                              CO.sub.2 selectivity in % based                                    Activity of the                                                                          on the amount of ethylene                               Example    Catalyst   reacted                                                 ______________________________________                                        1          3.2        9.6                                                     2          2.5        10.8                                                    3          2.4        11.4                                                    4          3.0        10.7                                                    5          2.8        10.0                                                    6          3.8        10.7                                                    7          1.6        8.6                                                     8          2.3        10.0                                                    Comparative                                                                              2.1        12.4                                                    ______________________________________                                    

To determine the activity of the catalyst, the temperature in the middleof the wall of the Berty reactor used for testing was recorded at aconstant oxygen conversion of about 45%. Low wall temperatures at aconstant oxygen conversion meant a relatively high catalyst activity.

Various modifications of the catalyst and processes of the invention maybe made without departing from the spirit or scope thereof and it is tobe understood that the invention is intended to be limited only asdefined in the appended claims.

What we claim is:
 1. In a process for preparing vinyl acetate in the gasphase from ethylene, acetic acid and oxygen and/or oxygen-containinggases in the presence of a catalyst, the improvement comprising using asupported catalyst comprising palladium and/or a compound thereof, goldand/or a compound thereof and also alkali metal compounds on aparticulate, porous, support, said catalyst comprising, based on thetotal mass of the catalyst, 0.2-2.5% by weight of calladium 0.2-2.5% byweight of gold and 0.1-10% by weight of alkali metal and which catalystis produced bya) impregnating the subject with a solution of solublepalladium and gold compounds, b) converting the soluble palladium andfold compounds into insoluble palladium and gold compounds by additionof an alkaline solution to the support, c) reducing the insolublepalladium and gold compounds on the support with a reducing agent in theliquid or gaseous phase, d) impregnating the palladium and goldcontaining support with at least one soluble alkali metal compound ande) finally drying the impregnated support at a maximum of 150° C.whereinthe catalyst is brought into contact with at least one peroxidiccompound in step b) as the catalyst.
 2. The process of claim 1 carriedout at temperatures of from 100 to 220° C. and pressures of from 0.1 to2.5 MPa.